As a next-generation wireless communication system for realizing high-speed and large-capacity communication, there is known LTE-Advanced (Long Term Evolution-Advanced) standardized by 3GPP (3rd Generation Partnership Project) that is a standards body. As described in NPL 1, in LTE-Advanced, provided is a heterogeneous network in which in a macro cell formed by a macro cell base station, a pico cell base station including a pico cell having a cell radius smaller than that of the macro cell is disposed. In general, a radius of a macro cell is several hundred to several thousand meters, and a radius of a pico cell is several ten to several hundred meters.
As illustrated in FIG. 13, for example, a wireless communication system 600 in a heterogeneous network includes a plurality of wireless base stations (for example, a macro cell base station 61 and a pico cell base station 62) and a wireless communication terminal (User Equipment) (hereinafter, referred to as a “UE”) 63. Each of the wireless base stations is communicable with an MME/SAE-GW 65 of a core network unit that is a host node of the wireless base stations via an S1 interface 64. In the above description, MME/SAE-GW is an abbreviation of Mobility Management Entity/System Architecture Evolution Gateway. Further, the respective wireless base stations are communicable with each other via an X2 interface 66. A macro cell 67 is a cell in which the macro cell base station 61 is communicable. A pico cell 68 having a cell radius smaller than a cell radius of the macro cell 67 is formed in the macro cell 67. The pico cell 68 is a cell in which the pico cell base station 62 is communicable. On the basis of the cell disposition described above, the pico cell 68 can accommodate the UE 63 connected to the macro cell 67. Therefore, a load of the macro cell base station 61 can be distributed. When the UE 63 is located in the macro cell 67, the UE 63 basically communicates with the macro cell base station 61, but when the UE 63 is further located in the pico cell 68, the UE 63 communicates with not the macro cell base station 61 but the pico cell base station 62.
Further, commonly, in a heterogeneous network, a UE compares reception quality in the UE of a wireless signal transmitted from a macro cell base station (hereinafter, referred to as “wireless quality of a macro cell”) with reception quality in the UE of a wireless signal transmitted from a pico cell base station (hereinafter, referred to as “wireless quality of a pico cell”), and is connected to a base station having higher wireless quality. However, in general, transmission power of the macro cell base station is higher than transmission power of the pico cell base station, and therefore, in many cases, wireless quality of a macro cell is higher than wireless quality of a pico cell. Therefore, a chance in which a UE is accommodated in the pico cell is decreased. As a result, a problem that a load of the macro cell base station is not sufficiently distributed may occur.
For the problem described above, NPL 2 proposes that in LTE-Advanced, cell selection based on CRE (Cell Range Expansion) is executed. In the cell selection based on CRE, the UE 63 adds an offset value to wireless quality of a pico cell. By addition of the offset value, the pico cell 68 is changed to a pico cell 69 in which a cell radius is expanded, as illustrated in FIG. 13. Thereby, a chance in which a pico cell is selected as a connection destination of the UE is increased, and therefore load distribution of a macro cell base station is expectable.
PTL 1 describes that a first throughput representative value representing throughputs between a first wireless base station and terminals and a second throughput representative value representing throughputs between a second wireless base station and terminals are determined. On the basis of the first throughput representative value and the second throughput representative value, coverage of the first wireless base station and coverage of the second wireless base station are adjusted.
Further, PTL 1 describes a wireless base station that adjusts coverage of the own station. On the basis of a throughput between a wireless base station and a terminal, a communication area range of the wireless base station is adjusted. Further, there is no direct involvement in which one of wireless base stations is selected.
Further, PTL 2 describes that a parameter received from the outside is added to/subtracted from a cell-ranking measurement value, and on the basis of the cell-ranking measurement value after addition/subtraction, a cell is ranked, and thereby a cell is reselected on the basis of the ranking.
Further, PTL 3 describes a technique in which a CIO (Cell Individual Offset) parameter is set to be a lower value when a femto cell is present in a cell center, and is set to be a higher value when the femto cell is present in a cell edge.